The penalty for an excess contribution is 6% of the excess. For TurboTax to come up with a $790 penalty it would have to be a $474 penalty for the $7,900 contribution and somehow a $316 penalty for something else. $316 would be the penalty for 8/12 of $7,900 = $5,267, but I don't know where that would have come from. It doesn't make sense that it would have come from a 2017 excess since the contribution limit for 2017 would have been some multiple of $7,750 / 12. Perhaps the entire $790 is from something other than an excess-contribution penalty since you say that you told TurboTax that you would remove the excess. Examine Forms 8889 and 5329 to see where the $790 is coming from.
